"It seems that Zune and Plays for Sure are closely related indeed. While the Zune player will not accept content from other Plays for Sure services it seems content from the Zune marketplace will work on other Plays for Sure compatible devices (it’s clearly not something supported and certainly not clear how long it will work)."

JupiterResearch (by ArsTechnica.com) is the source of this information but we still don't have the compatible players brands available. Sounds like  a bug in some player's DRM identification but in any case don't think Zune Marketplace content will continue to "Play for Sure". 

[via ZuneThoughts.com ] 

EDIT: ClicZune, we had forgotten this, although it was only some weeks ago! Thanks for the tip of on your comment:

We also asked David (David Caulton, a Zune team member) if he could confirm that Zune music will play on PlayForSure players. David said “Zune content can get out via Windows Media Player into playsforsure devices, but this scenario isn’t officially supported and may not be reliable.”